One prior art technique for introducing dopants into a semiconductor wafer is to direct an ion beam along a beam travel path and selectively position silicon wafers to intercept the ion beam. This technique has been used in ion implanters to dope the wafer with controlled concentrations of the ion material.
One example of a commercial ion implantation apparatus is the Eaton NV 200 Oxygen Implanter. This prior art ion implanter utilizes an oxygen ion source having a cathode that includes a filament for providing electrons for ionizing oxygen molecules. Electrons emitted by the cathode are accelerated through a region containing oxygen gas in controlled concentrations. The electrons interact with the gas molecules, yielding energy to and ionizing the molecules. Once ionized, the charged oxygen molecules are accelerated along a path of travel thereby forming an ion beam for semiconductor wafer implantation. An ion source utilizing a cathode filament is disclosed in U.S. Pat. No. 4,714,834 which issued in the name of Shubaly.
Alternate proposals for ion source construction include the use of a microwave ion source that does not require a cathode or cathode filament. A microwave-powered ion source excites free electrons within an ionization chamber at a cyclotron resonance frequency. Collision of these electrons with gas molecules ionizes those molecules to provide ions and more free electrons within the chamber. These ions are then subjected to an accelerating electric field and exit the chamber in the form of an ion beam. U.S. Pat. No. 4,883,968 to Hipple et al. discloses one such resonance ion source. The disclosure of the '968 patent to Hipple et al. is incorporated herein in its entirety by reference.
A key element in the performance of an ion implantation apparatus is how accurately the generated ion beam path corresponds to a desired predetermined beam travel path. Ions generated in the ionization or arc chamber exit the chamber through an elongated aperture or arc slit. The ions are accelerated along a beam path by an electric field generated by an energized extraction electrode located near the arc chamber. The extraction electrode is comprised of two semicircular disk halves which are spaced apart forming an elongated gap through which the ions travel.
The position and alignment of the arc slit and the extraction electrode gap is critical to achieving a beam path that coincides with the predetermined beam path. Both the arc slit and the extraction electrode gap must be positioned so as to be axially aligned with the predetermined beam path. In prior art ion implantation devices, accurate alignment of the arc slit with respect to the predetermined beam path was difficult because of cumulative positioning tolerances between the arc slit and an ion beam source housing.
The source housing supports the components associated with generating the ion beam. In prior art devices, the are slit is formed in an extraction member or plate which is mounted to the arc chamber. The arc chamber is supported by an ion source assembly. The ion source assembly is mounted within an ion source assembly support tube. The support tube is secured to an insulator. The insulator is coupled to the ion beam source housing. The cumulative tolerances associated with positioning the arc slit with respect to the ion beam source housing render the proper alignment of the arc slit with the predetermined beam path a difficult, costly and time consuming task. Furthermore, since the electrode plates are mounted on structure which is attached to and extends from to the ion beam source housing, accurate alignment of the arc slit and the extraction electrode gap was also difficult.
The present invention concerns a method and apparatus which overcomes the difficulties in positioning and aligning the arc slit and the extraction electrode gap with each other and the predetermined beam path.